Determinação das características seminais e o efeito de diluidores no resfriamento do sêmen em duas variedades de tilápia do Nilo

Abstract

Nile tilapia (Oreochromis niloticus) is the second most produced fish species in the world and, to further expand its production, the species has been the object of breeding programs. The GIFT (Genetically Improved Farmed Tilapia) strain was developed by successive selections and crosses seeking productive characteristics. The UFLA strain was formed after years of indirect selection for growth. However, few studies are conducted aiming to characterize and determine the semen of this species, to establish preservation protocols, and to enable artificial reproduction. The objective of this study was to determine and characterize the sperm and seminal plasma of GIFT and UFLA strains and to test the effect of extenders after sperm activation and during 24 hours of refrigeration. The experiments were conducted at the Federal University of Lavras using GIFT and UFLA broodstock males. For the first experiment, sperm was collected from 14 males (per strain) and immediately analyzed to determine sperm volume, spermatic concentration, membrane integrity, subjective spermatic motility rate, subjective velocity score and motility duration. Using the CASA system (Computer Assisted Sperm Analysis), the spermatic motility rate, curvilinear velocity (VCL), straight line velocity (VSL), average path velocity (VAP), and beat cross frequency (BCF) were evaluated. The pH, osmolality, and ions (Na + , Ca 2+ , K + ) concentration were evaluated to determine the seminal plasma. In the second experiment, the semen was divided into the following treatments: undiluted control, and diluted in NaHCO 3 , Na 3C6H5O7 , NaCl, KCl, CaCl2 and glucose solutions, and subjectively evaluated during 24 hours of cooling storage for spermatic motility rate and velocity score. For the third experiment, sperm of 5 males (per strain) was collected, diluted, and evaluated under the CASA system to determine motility rate and VCL during 120 seconds post-activation (SPA). Statistical analyses were carried out using the R software, and the data were submitted to ANOVA, followed by the Scott-Knott test, where relevant. For the first experiment, there was no significant difference between the strains for any of the quality parameters evaluated in fresh sperm and seminal plasma. The means for fresh sperm were of 0.76-0.70 mL of volume, 1.2-1.8 x 10 9 spermatozoa mL -1 , 76% of intact sperm, 86-88% of subjective motility, 4.8-5.0 of velocity score, and 1072-1163 s of motility duration. The CASA analyses presented means of 69-77% of motility, 57-65 µm s -1 of VCL and 18-20 Hz of BCF. The means for seminal plasma were of 260-273 mOsm kg -1 of osmolality, 8.0 of pH, 149.6 mmol L -1 of Na + , 0.7 mmol L -1 of Ca 2+ and 2.5 mmol L -1 of K + . For the second experiment, the undiluted control sample dried, allowing only the analysis at time 0 h. One hour after dilution, samples of all treatments yielded motility rate and subjective velocity higher than 50% and 3.1 for the GIFT strain and higher than 60% and 3.9 for the UFLA strain. Six hours after dilution, sperm samples diluted in solutions containing Na + and glucose presented motility rate and subjective velocity score above of 30% and 1.0 for the GIFT strain and for the UFLA strain of 60% and 2.2. For the third experiment, no differences were observed between treatments for the motility rates of both strains, at 10 SPA. After 30 s SPA, motility rates were higher than 50% were observed for all treatments except for those samples diluted in NaHCO3 for the GIFT strain. For the UFLA strain, only the undiluted control and diluted in KCl and glucose samples presented motility above 50%. Regarding the VCL, the UFLA strain maintained speeds longer than the GIFT strain. In conclusion, the selection of GIFT males for rapid growth did not affect the reproductive parameters of sperm in natura when compared to the UFLA strain. Sperm of UFLA males is more resistant to cool storage. Solutions NaCl and glucose can be used as extenders for the semen of O. niloticus.